3 Answers
3

You can upgrade to as high as 8GB with the 13" macbook pro, although 2x4GB SODIMMs would be pretty pricey. 2x2GB SODIMMs should work fine. You want PC3-8500 (1066Mhz) DDR3 SODIMMs.

You should be able to mix and match any sizes of chips, so you could buy a 4GB SODIMM and replace one of your 1GB for a total of 5GB, or else buy 2x2GB and replace both of your 1GBs for a total of 4GB. That would be cheaper.

DDR3 is for the latest Mac line only. Unibody Macs in 2008 and earlier used DDR2. Check compatibility with your vendor before you order. I recommend checking crucial.com to find out what spec you need, then buy from whatever vendor gives you the best deal.
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Kio DaneNov 30 '09 at 18:37

What you want really depends if Apple shipped your laptop with 2x 1GB sticks or 1x 2GB sticks. I think the only way to find out is to find your original invoice / specification or just open it up and take a look.